1. Field of the Invention
The invention relates to twin screw extruders and more particularly to an extruder for continuous processing of PVC powders.
2. Description of the Related Technology
Twin screw extruders are continuously operating screw presses with two meshing screws and are used to plasticize and extrude plastics. Twin screw extruders are used preferably for the continuous processing of PVC powder formulations.
Twin screw extruders may be divided into different zones which are distinguished. A melting zone, a mixing zone, a degassing zone and a discharge zone may be arranged in line from an extruder inlet to the outlet. As a rule, the screws have different configurations adapted to the particular zone, over their length. The screw turns may have different pitches and spacing in the zones and may be provided with interruptions, baffles or the like. The screw turns may be replaced by meshing toothed disks or kneading blocks particularly in the melting and/or the mixing zones in order to obtain a particularly intensive mixing of the powder in the melt and to homogenize the melt. The screws usually have continuous screw turns in the discharge zone which mesh with slight roll and flank clearances, thereby preventing any back flow of the melt and producing the necessary pressure for pressing the melt into a molding tool attached to the extruder.
Twin extruders of any type, equipped with cylindrical, conical or double conical screws, have the following problem in common: the melt is exposed to a very high shear force in the wedge or lens shaped overlapping area of the screws, in which the screws turns mesh with slight roll and flank clearances. The melt will be plasticized stronger in this zone and be at a higher temperature and have a lower viscosity than in the other cross-sectional areas of the extruder. Furthermore, the material is driven forward stronger in this overlapping area. The material therefore enters the tool attached to the extruder from the overlapping area at a higher temperature, lower viscosity and a higher driving force, than from the other cross-sectional areas. This results in a loss of quality in the extruded products. This problem leads to variations in PVC tubing wall thickness and corrugations of the internal side of the tubing. Undesirable surface waviness, differences in thickness, flow marks, etc., may result in PVC sheet production. For this reason, PVC foil usually cannot be produced with twin screw extruders. Planetary roll extruders are required, the cost of which amounts to a multiple of that of twin screw extruders. Extruders are frequently operated at only 50% of maximum capacity in the extrusion of PVC profiles. The aforedescribed inhomogeneity of the melt is less pronounced with lower rotating velocities.
Attempts were made to resolve the problem by special adaptations or corrections of the molding tool attached to the extruder, in particular so that the hotter melt from the overlapping area is distributed in different cross section areas of the mold. Also, various eating devices have been mounted on the mold in order to obtain a more uniform temperature distribution of the melt by supplemental heating. All of these measures on the side of the tool were found to be unsatisfactory.